1. Field of the Invention
The present invention relates to a concrete cask suitable for transportation and long-term storage of radioactive materials, such as spent nuclear fuel. More particularly, the invention is concerned with technology for increasing the performance of the concrete cask through improvements of its cement composite and construction.
2. Description of the Related Art
Conventional technologies concerning concrete casks are disclosed in Japanese Unexamined Patent Publication Nos. 2001-141891 and 2001-305375, for example.
Patent Publication No. 2001-305375 describes an construction of a concrete cask, in which surfaces of the concrete cask are covered by steel plates to prevent its cracking and loss of water content and to achieve enhanced durability, and partitions are provided in the cask to prevent concrete cracking.
Patent Publication No. 2001-141891 describes a concrete cask manufactured by using an ordinary low heat-resistance concrete blend. According to the Publication, there is formed a gap between the concrete cask and a canister taking into consideration temperature limitations of concrete. The concrete cask has air inlets and air outlets. External air introduced through the air inlets is caused to convent inside the cask to directly cool the canister and avoid temperature increase of the concrete. To further decrease the temperature of the concrete, the concrete cask is provided with liners and internal fins.
While the technology of Publication No. 2001-305375 helps prevent concrete cracking, the concrete cask has poor heat resistance. It is therefore necessary to continuously introduce external air into the cask. An opening for introducing the external air acts as a “shielding defect” which could increase streaming of radioactive rays. The external air is likely to contain corrosive substances, such as chlorides, which would accelerate corrosion of a steel-made canister. Therefore, the concrete cask of this prior art technology tends to pose problems related to sealing performance.
Since the concrete cask of Publication No. 2001-141891 is made of concrete having low heat resistance, it is necessary to introduce air to the interior of the cask. This would require a complicated structure of the concrete cask and pose a risk of releasing radioactive gases in case of canister breakage.
The conventional concrete casks is made of concrete of which heat-resisting temperature is 90° C. or less, so that they generally necessitate complicated structures such as the provision of through holes for ventilation. If such through holes are made in the concrete cask, the external air goes into direct contact with the canister provided inside the cask, resulting in corrosion of the canister and accidental release of radioactive material into the environment from inside the canister. Conventional concrete retains hydrogen, which is effective for shielding neutrons, in the form of free water which evaporates at temperatures of 100° C. or above. Thus, the conventional concrete casks have a problem that their neutron-shielding performance considerably decreases when used under high-temperature conditions (100° C. or above).